1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and particularly relates to a technology for forming an element isolation film.
2. Related Art
Conventionally, a region for forming a transistor and an element isolation region are created on a semiconductor substrate. The element isolation region is formed as described below, for example. As shown in FIG. 4A, a thin film 901 is provided so as to cover a region for forming the transistor on a semiconductor substrate 900, and further, as shown in FIG. 4B, a silicon nitride film 902 is provided on the thin film 901. The thin film 901 is one that is harder than a silicon nitride film 902. Thereafter, as shown in FIG. 4C, the semiconductor substrate 900 is etched through a mask of the silicon nitride film 902 to form a trenched portion 904. As shown in FIG. 4D, the trenched portion 904 is filled, and an element isolation film 905 that covers the silicon nitride film 902 is formed. Thereafter, as shown in FIG. 4E, the element isolation film 905 is polished via a chemical mechanical polishing (CMP) until the silicon nitride film 902 is exposed to remove the silicon nitride film 902 and the thin film 901 via a wet etch process, as shown in FIG. 4F (see, for example, Japanese Patent Laid-Open No. 2004-172,417). Since the silicon nitride film 902 functions as a stop film for the chemical mechanical polishing process during the polishing of the element isolation film 905, a height dimension of the element isolation film 905 from the surface of the substrate is substantially equivalent to a height dimension of a combination of the thickness of the silicon nitride film 902 and the thickness of the thin film 901, or shorter than a height dimension of a combination of the thickness of the silicon nitride film 902 and the thickness of thin film 901.
However, there is a room for improvement in view of the following points in the technology described in Japanese Patent Laid-Open No. 2004-172,417. The devices in recent years include an element isolation film for separating a transistor from another transistor, and a width dimension of the element isolation film is decreasing. Corresponding to such trend of providing a reduced width of the element isolation film, a trenched portion, which have conventionally had a tapered cross-section, is required to have a substantially rectangular cross section, in which a width of a bottom surface or a lower side is substantially equivalent to a width of an aperture or an upper side. In order to form such trenched portion having the above-described geometry, it is required that a substrate is vertically etched, and that precipitates created in the process for forming the trenched portion via the etch process is reduced. The precipitates created in the etch process cover the silicon nitride film, so as to function as a mask for adjusting an amount of an etched silicon nitride film created in the etch process for forming the trenched portion. Therefore, a decrease in the amount of the precipitates causes an increase in an amount of the etched silicon nitride film.
Since the element isolation film is more easily polished than the silicon nitride film, when the silicon nitride film that is thinner in thickness is employed as a stop film for a CMP process, a level of the element isolation film 905 may possibly be lower than a level of a surface of the substrate 900, as shown in FIG. 5. Further, in order to adjust the height dimension of the element isolation film from the surface of the substrate to a certain dimension, an adjustment process for the height of the element isolation film may be conducted after the silicon nitride film is removed. When the silicon nitride film that is thinner in thickness is employed as a stop film for a CMP process, the height dimension of the element isolation film from the surface of the substrate is reduced, so that an adjustment of the height dimension of the element isolation film from the surface of the substrate would be difficult.
Further, an increase in the amount of the etched silicon nitride film leads to an uneven etching of the silicon nitride film, so that it is difficult to obtain an element isolation film having an even and uniform height dimension. When a well is formed in the silicon substrate, an impurity is generally implanted into a lower portion of the element isolation film in the silicon substrate. But in such case, a variation in the height dimension of the element isolation film causes a difficulty in providing a well which has a desired depth.